Dinosaurs with Marionette Jaws: The Case for the Predentary in Ornithischians

As I mentioned in a previous post, ornithischian dinosaurs are enormously diverse. From the horned ceratopsians and dome-headed pachycephalosaurs to the armored stegosaurs and ankylosaurs and duck-billed hadrosaurs (and others), they make up a majority of the herbivorous non-avian dinosaur taxa to have ever lived. It’s a well-known fact that the presence of a single predentary bone at the front of the lower jaw is THE absolute dead give-away that a dinosaur is a member of Ornithischia; even better than noting their namesake “bird-like hip”. But have you ever wondered exactly why ornithischians had it in the first place? And, better yet, why is it that it persisted throughout ~140 million years of ornithischian evolution? This unique bone is largely unseen in a majority of vertebrates (other than very few convergences in fish, amphibians, and extinct birds), and yet this hugely diverse group of dinosaurs had it and kept it for their entirety.

I first started thinking about the enigmatic predentary bone for my undergrad senior thesis in 2008-2009. From then, I carried on looking into it as part of my PhD dissertation. I’ve been fascinated by both its huge diversity in shape as well as function throughout ornithischian groups. That’s what led to our new paper just out in Anatomical Record, “The predentary bone and its significance in the evolution of feeding mechanisms in ornithischian dinosaurs” (Nabavizadeh and Weishampel, 2016). It’s a good old-fashioned comparative anatomy project examining the form and function of this bone and its influence in the herbivorous lifestyles of these incredible creatures. My previous paper (Nabavizadeh, 2016) looked at the differences in mechanical advantage in the overall jaw structure across ornithischian taxa. Our new paper goes more in depth into the anatomy of the jaw as it relates to the predentary. What did we find? Well, there are actually (broadly) three types of predentary joints!
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The first morphotype is the most simplistic, the predentaries resting against the front ends of the dentaries on each side of the jaw without much of a clasping, tight junction. A majority of the more basal ornithischians as well as basal ornithopods and ceratopsians, and it allows for very slight long-axis rotation of the lower jaw on each side of the mandible. The second morphotype involves the development of a symphyseal process (downturned front end of the dentaries) with a loosely attached predentary resting against or enveloping the dentaries, with a lot of wiggle room. Interestingly, this is convergently seen in hadrosaurs as well as ankylosaurs and stegosaurs and allows for a larger range of long-axis rotation. Lastly, the third morphotype involves a tightly clasping junction between the predentary and each of the dentaries, restricting any long-axis rotational movement. This is primarily seen in more advanced ceratopsians, especially the ceratopsids.

Why should you care? Well, we all like to think of many of these dinosaurs as enormous analogs to our present day herbivorous mammalian fauna, primarily due to the fact that they are “chewing” their food. But what we often don’t hear about is the fact that they are processing food in a completely unique way. In many of us mammals, do you notice how we usually chew on one side of the mouth at a time, with all the muscle forces from both sides focusing on that one bite point? Well, these dinosaurs were likely able to chew on both sides of the jaw at the same time, with the predentary sort of blocking the stresses from going to the opposite side of the jaw. In the cases of ankylosaurs, hadrosaurs, and ceratopsians, palinal feeding takes place, where the jaw is pulled backward during the feeding stroke. Additionally, ankylosaurs and hadrosaurs both incorporate a major component of long-axis rotation of the mandibles in addition to the palinal feeding stroke due to the loose connection of the predentary with the dentaries. This created a bolt-cutter-like mechanisms that would have been incorporated into their feeding, pushing vegetation straight into the mouth in every chew. Of course, this joint is formed by a fibrocartilaginous capsule, but this would allow some degree of rotation to occur. And, with the presence of a symphyseal process in the larger animals, the rotation of the mandibles at the front of the jaw are allowed to rotate much more freely and not separate during rotation, whereas more basal ornithischians without symphyseal processes would have this problem with too much rotation, although they still had wiggle room. The bigger you get, the more you need a way to rotate the dentary further in order to fulfill the feeding stroke. As for the ceratopsians, thy were primarily focused on orthal and palinal movements, but with their shearing tooth row morphologies, rotation of the mandible would not have been as necessary.

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If it wasn’t for the predentary, the two dentaries would easily disarticulate with much rotation. The predentary would have acted as some sort of bracing axial point against which the dentaries could rotated and move around. Think of it as something like a marionette puppet, with multiple components forming the entire jaw with movement possible between each component. Various forms of a ball-to-socket jaw joint, a recurved coronoid process, and curved tooth rows also corroborate with these kinds of mechanisms; however, the greatest evidence of all has been with microwear studies that show us more and more about their paleoecology. It’s always super exciting to see data on tooth wear, and I’m looking forward to seeing more studies integrating all aspects of the feeding mechanisms to get the whole picture! Until next time!

Reference:

Nabavizadeh, A., and Weishampel, D. B. The predentary bone and its significance in the evolution of feeding mechanisms in ornithischian dinosaurs. The Anatomical Record. DOI: 10.1002/ar.23455

Link: http://onlinelibrary.wiley.com/doi/10.1002/ar.23455/abstract

~ Ali

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